Saturday, November 17, 2012

Applications - Selection of Deep Foundations.

Driven pile groups are typicallyused by the Corps  of  Engineers to support locks, dry docks, and other facilities constructed  in river systems, lakes, lagoons, and other offshore applications.   Drilled shafts typically support many permanent onshore structures such as administrat ive buildings, warehouses, dormitories, and clinics.    Drilled  shafts are divided into two groups: displacement and mnondisplacement.

(1)  Displacement.  Dri ven pile foundations are usually preferable in  loose, cohesionless, and soft soils, especially where excavations cannot  support fluid concrete and where the depth of the bearing stratum is uncertain. Groundwater conditions can be a deciding factor in  the selection of driven piles rather than drilled shafts.  Uncased shafts  are  generally excluded  from consideration where artesian pressures are  present.  Often more than one type of driven pile may meet all requirements  for a particular structure.  Driven piles according to their application are presented in Figure 1-6.

(a)  Figures 1-6a and 1-6b illustrate  piles classified according to their behavior   as end-bearing or friction piles.  A pile embedded a significant length into stiff clays, silts, and dense sands without significant end bearing resistance is u sually a friction pile.  A pile driven through relatively weak or compressible  soil to an underlying stronger soil or rock is usually an end-bearing pile.

(b)  Piles designed primar ily to resist upward forces are uplift or tension piles (Figure 1-6c), and the resistance to the upward force is by a combination of side (skin) friction and self weight of the pile.

(c)  Lateral forces are r esisted either by vertical piles in bending (Figure1-6d) or by batter piles or groups of vertical and batter piles (Figure 1-6e).

(d)  Piles are used to transfer lo ads from above water structures to below the scour depth (Figure  1-6f).  Piles are also used to support structures that may be endangered b y future adjacent excavations (Figure1-6g).  In order to prevent undesirable movements of structures on shrink/swell soils, a pile anchored as shown in Figure 1-6h can be used.

(2) Nondisplacement. Drilled shafts are especially suitable for supporting large co lumn loads of multistory structures and bridge abutments or piers.  They are suitable for resisting large axial loads and lateral load s applied to the shaft butt (top or head) resulting from  wind forces; these are also used for resisting uplif t thrust applied to the shaft perimeter through soil-shaft  interface friction and from heave of expansive soil.  Figure 1- 7 illustrates examp le load ranges for drilled shafts in different soils.  The loads shown  are for guidance only and can vary widely from site to site . Cylindrical shafts  are usually preferred to underreamed ones because of ease in construction and ease in inspection.  Table 1-5 provides further details of
the  applications, advantages, and disadvantages of drilled shafts.  Other aspects of drilled shafts include:

(a)  Drilled shafts may  secure much or all of their vertical load capacity from frictional side resis tance (Figure1-7a).  An enlarged base using a bell or underream may also increase the vertical load capacity, provide uplift resistance  to  pullout  loads, an  resist  uplift  thrust  from heave of expansive soil. Shafts subject to pullout loads or uplift thrust must have  sufficient reinforcement steel to absorb the tension  load in the shaft and sufficient skin friction  and underream resistance to prevent shaft uplift movements.

(b)  The shaft may pass through relatively soft, compressible deposits and develop vertical load capacity
from end bearing on hard or dense granular soil (Fig. 1-7b) or rock (Fig. 1-7c).  End-bearing capacity should be sufficient to support vertical loads supplied by the structure as well as any downdrag forces on the shaft perimeter caused by negative skin friction from consolidating soil (Fig. 1-7b).

(c) Single drilled shafts may be constructed with large diameters, typically 10 feet or more, and can extend to depths of 200 feet or more.  Drilled shafts can be made to support large loads and are seldom constructed in closely spaced groups.

(d) Drilled shafts tend to be preferred compared with driven piles as the soil becomes harder.  Pile driving
becomes difficult in these cases, and the driving vibration can adversely affect nearby structures.  Also, many onshore areas have noise control ordinances which prohibit 24-hour pile driving (a cost impact). 

(e)  Good information on rock is required when drilled shafts  are supported by rock.  Drilled shafts placed in weathered rock or that show lesser capacity than expected may require shaft bases to be placed deeper than anticipated. This may cause significant cost overruns.

Figure 1-6.  Driven pile applications

Figure 1-7.  Load resistance of drilled shafts in various soils.

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